Semiconductor apparatus, method for growing nitride semiconductor and method for producing semiconductor apparatus

ABSTRACT

A semiconductor apparatus includes a substrate made of a diboride single crystal expressed by a chemical formula XB 2 , in which X includes at least one of Tl, Zr, Nb and Hf, a semiconductor buffer layer formed on a principal surface of the substrate and made of Al y Ga 1-y N (0&lt;y≦1), and a nitride semiconductor layer which is formed on the semiconductor buffer layer and which includes at least one kind or plural kinds selected from among 13 group elements and As.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a semiconductor apparatus and specificallyrelates to a semiconductor apparatus, a method for growing a nitridesemiconductor and a method for producing a semiconductor apparatus whichare suitable for a light emitting device and a light receiving devicesuch as a light emitting diode (LED), a laser diode (LD), a solar celland a photosensor, and an electronic device such as a transistor and apower device.

2. Description of the Related Art

A nitride semiconductor containing Ga as a main constituent (a GaNsemiconductor) is utilized as a material for an optical device such as alight emitting diode of blue light or violet light, a laser diode and aphotodetector. Moreover, attention is given to the GaN semiconductor asa high-performance material for an electronic device as well, becausethe GaN semiconductor is capable of satisfying high frequency and highelectric power, and is highly reliable.

Further, a light emitting diode using the GaN semiconductor is known(refer to Japanese Unexamined Patent Publication JP-A 4-321280 (1992),for example). An example of a structure of such a light emitting diodeis shown in FIG. 17. A GaN buffer layer 21 is formed on a sapphire(Al₂O₃) substrate 20, and on the GaN buffer layer 21 is formed a growthlayer made of a GaN semiconductor layer having a multilayer structuremade by sequentially laminating an n-GaN layer 22 of the n-typesemiconductor layer, an n-AlGaN cladding layer 23 of the n-typesemiconductor layer, an InGaN light emitting layer 24, a p-AlGaNcladding layer 25 of the p-type semiconductor layer and a p-GaN layer 26of the p type semiconductor layer. In part of the growth layer, a regionfrom the p-GaN layer 26 to (an upper portion of the n-GaN layer 22) isetched and removed, and thus part of the n-GaN layer 22 is exposed. Then-type electrode 20 is formed on an upper surface of the exposed region,and the p-type electrode 27 is formed on an upper surface of the p-GaNlayer 26 of an uppermost layer.

Since production of a single crystal substrate of the GaN semiconductoris difficult, there is a need to form a semiconductor apparatus usingthe GaN semiconductor on a substrate made of a different material.Although sapphire is generally used as the substrate 20, a Si substrate,a ZnO substrate, an MgO.Al₂O₃ (spinel) substrate, a SiC substrate, aGaAs substrate and the like are tested other than the sapphiresubstrate.

In the case of growing the GaN semiconductor layers on the sapphiresubstrate 20, a problem is a lattice mismatch between them. A relationof lattice constants thereof is as shown below. GaN grows in a directionrotated through 30° from an a axis on a c plane of the sapphiresubstrate. Regarding sapphire, a lattice constant a (which is describedas a₁ in Table 1 described later) is 4.7580 Å. An interval value (whichis described as a₂ in Table 1 described later when a lattice rotatesthrough 30° is 2.747 obtained by 4.758×1/1.732 (a numerical valueobtained by multiplying a length of the a axis in a unit lattice ofsapphire by 1/1.732 becomes a reference). On the other hand, regardingGaN, a lattice constant a is 3.1860 Å.

A ratio of the lattice mismatch of GaN with reference to sapphirebecomes +15.98% (=100×(3.1860−2.747)/2.747). Thus, the lattice constantof sapphire is significantly different from the lattice constant of GaN.Consequently, a good quality crystal cannot be obtained when GaN isdirectly grown on sapphire. Besides, it is possible to consider asubstrate made of a material of another kind in the same manner.

In a prior art, in order to increase crystalline quality of the growthlayer, a buffer layer made of an AlN or GaN material which is amorphousor polycrystalline is formed on the (0001) plane of the sapphiresubstrate 20 in advance, and the GaN growth layer is formed on thebuffer layer. The buffer layer has a function of reducing the latticemismatch between the GaN growth layer and the sapphire substrate andincreasing crystalline quality.

Further, in the case of a semiconductor apparatus such as a laser diodeor a transistor which requires a better quality crystal, after the GaNsemiconductor layer is once grown on the single crystal substrate 20,the single crystal substrate 20 is eliminated, and then thesemiconductor apparatus is formed. This is because when thesemiconductor apparatus is formed on a substrate 20 made of a differentmaterial from the semiconductor apparatus, a crystal defect whichresults from a difference in coefficient of thermal expansion occurs ina cooling process after crystal growth at high temperatures of 1000° C.or more.

Furthermore, in another prior art, there is also a method for growingthe GaN semiconductor layer 22 in which a mask made by patterning a SiO₂thin film is formed and the GaN semiconductor is grown on the mask in alateral direction, in order to avoid an influence by the latticemismatch with the substrate 20.

However, since the ratio of the lattice mismatches between the sapphiresubstrate 20 and the GaN layer 22 is as large as 15.98%, the GaN growthlayer 22 contains dislocations whose density is 10⁸ to 10¹¹ cm⁻² evenwhen formed via the buffer layer 21 made of an AlN or GaN material.Moreover, even a layer of a GaN crystal laterally grown afterelimination of the sapphire substrate 20 contains dislocations whosedensity is 10⁴ to 10⁷ cm⁻². They contain extremely large dislocations ascompared with GaAs grown on a GaAs substrate containing dislocationswhose density is 10² cm⁻² to 10⁷ cm⁻².

The dislocation density of the GaN growth layer significantly restrictsperformance of a semiconductor apparatus to be produced from this, andmoreover, there is a need to increase the amount of additive elements inthe semiconductor layer for generation of sufficient carrier. This has aproblem of deteriorating a characteristic of a semiconductor apparatus,such as a life, a withstand voltage, a driving voltage, consumedelectric power (operation efficiency), an operation speed or a leakcurrent.

Then, in a proposed art, growth of a nitride semiconductor on a diboridesingle crystal substrate expressed by a chemical formula XB₂, in which Xcontains at least one of Ti and Zr, is proposed. TABLE 1 Coefficient ofLattice constant thermal expansion [Å] [×10⁻⁶/K] ZrD₂ 3.1696 5.9 TiD₂3.0303 Sapphire(Al₂O₃) a₁ = 4.7580, a₂ = 2.747 7.5 CaN 3.1860 5.6 AlN3.1114 4.2 BN 2.5502 InN 3.54 5.7 SiC 3.08 4.3 GaAs 4.00 8.7 Si 3.84 2.6

Since the nitride semiconductor is formed with a good lattice matchrelation with the diboride single crystal substrate in this manner, alattice defect in the growth layer is small, and crystalline quality ofthe nitride film is extremely good.

However, in the aformentioned prior art, for example, when GaN iscrystal-grown as the nitride semiconductor on the diboride singlecrystal substrate, because of a change of a growth temperature in agrowth process, B in the substrate diffuses into the crystal-grown GaNcrystal, and a nitride semiconductor GaBN which contains a ternary 13group (a former IIIB group element) is generated on an interface betweenthe GaN and the substrate. In the case of RN, as shown in Table 1, amismatch of lattice constants becomes as large as approximately 20%, ascompared with GaN (for example, (2.5502-3.1696)/3.1696-19.5%).Therefore, in the case of CaBN, which is a ternary nitridesemiconductor, a difference of lattice constants becomes significantlylarge as a mixed crystal ratio of B becomes large, unlike AlGaN, whichis a ternary nitride semiconductor which the mismatch ratio of latticeconstants is 2% or less. Consequently, a lattice defect occurs on theinterface even when GaN is grown on the diboride single crystalsubstrate as described above, and a good quality crystal cannot beobtained.

In recent years, research and development of a nitride semiconductorwhich contains at least one selected from among B, Al, Ca, In and Tlhave become active, and applied technique has rapidly developed. Then,at present, a light emitting diode of green, blue and ultraviolet, alaser diode of blue and violet, and the like using the nitridesemiconductor is put to practical use.

In specific, (InN)_(x)(GaN)_(1-x) whose band gap covers from red toviolet and (AlN)_(x)(GaN)_(1-x)N whose band gap covers from violet toultraviolet are positioned as main materials among III group nitridesemiconductors, because the former makes it possible to realize a deviceemitting light of blue green, blue, violet or the like which was notrealized before, and the latter makes it possible to expect applicationas a light source for measurement, sterilization or excitation.

The III group nitride semiconductor is grown from vapor phase on asingle crystal substrate made of sapphire, SiC, GaAs, Si or the like bya MOVPE (Metalorganic Vapor Phase Epitaxy) method.

The III group nitride semiconductor is a hexagonal symmetry, and a-axislattice constants of InN, GaN and AlN are 0.311 nm, 0.319 nm and 0.354nm, respectively. Moreover, respective lattice constants of(InN)_(x)(GaN)_(1-x) and (AlN)_(x)(GaN)_(1-x) are values in a middlerange of the respective aforementioned lattice constants of InN, GaN andAlN depending on x.

However, since intervals between atoms that sapphire, SiC, GaAs and Sishould achieve lattice matches with the III group nitride semiconductorare 0.275 nm, 0.308 nm, 0.400 nm and 0.384 nm, respectively (refer toTable 1), and substrates which completely achieve lattice matches arenot obtained.

In the meantime, in another prior art a technique on a low-temperaturebuffer layer is proposed (refer to Japanese Examined Patent PublicationJP-B2 4-15200 (1992) and Japanese Patent 3026087, for example).

It is possible to grow a good quality crystal on the lattice mismatchingsubstrates described above by using these technique, however,penetration dislocations of approximately 10⁸ cm⁻² to 10¹¹ cm⁻² stillexitss. Moreover, differences in the coefficient of thermal expansionbetween the above single crystal substrates and the nitridesemiconductor are large, and a difference in contraction amounts aftercrystal growth at a high temperature of approximately 1000° C. causescracks.

As a method for solving these problems, in another prior art a techniqueof growing a nitride semiconductor on the (0001) plane of a ZrB₂ singlecrystal substrate is proposed (refer to Japanese Unexamined PatentPublication JP-A 2002-43223).

The ZrB₂ single crystal substrate is a hexagonal symmetry, and a latticeconstant of an a axis is 0.317 nm (refer to table 1), which completelyachieves a lattice match with 0.26 of x of (AlN)_(x)(GaN)_(1-x).Moreover, the coefficient of thermal expansion is 5.9×10⁶K⁻¹, which is aclose value to 5.6×10⁻⁶K⁻¹ of that of GaN.

Further, the ZrB₂ single crystal substrate, whose resistivity is assmall as 4.6 ηΩ·cm, is electrically conductive. On the other hand, asapphire substrate 110 generally used as a substrate up to now isinsulative, and therefore, a light emitting diode formed on the sapphiresubstrate has a structure such that two electrodes 101 and 109 aredisposed on the same plane side (the upper side of the substrate 110 inFIG. 18) as shown in FIG. 18.

The structure shown in FIG. 18 is a structure in which a low-temperaturebuffer layer 107, the n-type contact layer 106, the n-type claddinglayer 105, a light emitting layer 104, the p-type cladding layer 103 andthe p-type contact layer 102 are sequentially laminated on a sapphiresubstrate 110, and furthermore, a p electrode 101 is formed thereon.Moreover, an n electrode 109 is formed on an exposed surface of then-type contact layer 106.

As described above, in the last one or two years, research anddevelopment of the technique of growing the nitride semiconductor on theZrB₂ single crystal substrate have progressed.

According to “Abstr. 13th Int. Conf. Crystal Growth, August 2001,02a-SB2-20”, a technique of enabling growth of GaN on the (0001) planeof the ZrB₂ single crystal substrate by an MBE method is proposed.

However, this technique has a problem that it is inferior in massproduction because it uses the MBE method.

Further, a technique of growing GaN on the (0001) plane of the ZrB₂single crystal substrate by using an AlN buffer layer by the MOVPEmethod is also proposed (refer to Ext. Abstr. (62nd Autumn Meet. 2001);Japan Society of Applied Physics, 12p R-14).

Further, in the prior art, a GaN film grown on the (0001) plane of aZrB₂ single crystal substrate by an MOVPE method has a problem that asurface shape thereof tends to become uneven as shown in FIG. 13.

As described above, in the case of growing a nitride semiconductor onthe (0001) plane of the ZrB₂ single crystal substrate by using an AlNbuffer layer, it is desired to reduce unevenness appearing on a surfaceshape thereof.

However, according to both the techniques, in the GaN film grown on the(0001) plane of the ZrB₂ single crystal substrate, a rocking curve halfvalue width of (0002) plane omega scan by an X-ray diffraction method,which becomes an indicator of evaluation of quality, is approximately1000 seconds, which is not sufficiently good (refer to “Study on thecrystal growth and properties of group-III nitride semiconductors onZrB₂ substrate by metalorganic vapor phase epitaxy” master's thesiswritten by Yohei Yukawa, graduate school of Meijo University, 2001).

Furthermore, since AlN is insulative, when the light emitting diode orthe like with the structure as shown in FIG. 8 as described later isproduced, resistance from the nitride semiconductor layer to thesubstrate becomes high, and an operation voltage becomes high.

Still further, a band gap of AlN is as large as 6.2 eV, and therefore,it is difficult to decrease resistance by doping.

In the aforementioned prior art, when the nitride semiconductor is grownon the (0001) plane of the ZrB₂ single crystal substrate, resistancefrom the nitride semiconductor layer to the substrate becomes high, andthe operation voltage becomes high.

In recent years, a nitride semiconductor such as gallium nitride (GaN),indium nitride (InN) or aluminum nitride (AlN) is used as a material foran optical device such as a light emitting diode of blue light or violetlight, a laser diode or a photodetector, because the nitridesemiconductor is a compound semiconductor of direct transition type, andhas a wide band gap.

Further, since the nitride semiconductor is capable of satisfying highfrequency or high electric power, and is highly reliable, the nitridesemiconductor is noted as a high-performance material for an electronicdevice.

Up to now, there is no substrate that achieves a lattice match with thenitride semiconductor. In a conventional art, the nitride semiconductoris grown by the use of a substrate made of a material, such as asapphire substrate different kind from nitride semiconductor.

However, for example, regarding the sapphire substrate and GaN, a ratioof lattice mismatch is 13.8% and a difference in the coefficient ofthermal expansion is 3.2×10⁻⁶/K, and there is a problem resulting fromthe mismatch such that dislocations of 10⁸ to 10¹⁰ cm⁻² arises in theGaN film because of a crystal defect caused on an interface between thesapphire substrate and the GaN film.

Further, because of the defect and thermal distortion, the GaN film iswarped, and crystalline quality is significantly deteriorated.

Furthermore, considering production of a device such as a laser diode,the nitride semiconductor is formed on a substrate made of a material ofdifferent kind from the nitride semiconductor such as GaN and the like,and therefore, there arises such a problem that, in the case of forminga reflection surface of a laser resonator, cleaved planes of thesubstrate made of a material and the nitride semiconductor aredifferent, and that formation by cleavage is difficult. Accordingly, agood quality nitride semiconductor substrate has been expected.

However, regarding the nitride semiconductor such as GaN, a meltingpoint is high and a dissociation pressure of nitride is high at themelting point, and therefore, production of a bulk single crystal isdifficult. Consequently, as described before, for example, by growing athick film of GaN on the sapphire substrate and then separating thesapphire substrate, which is made of a material of different kind fromGaN, and the GaN thick film, the nitride semiconductor apparatus isproduced.

However, in the step of separating the substrate and the nitridesemiconductor in the aforementioned production method, a method ofabrading the substrate arises a problem that stress from the nitridesemiconductor thick film becomes large as the substrate becomes thin,and that the stress acts on the substrate, thereby worsening a warpthereof and causing cracks.

In the meantime, as prior art another separation method, there is, forexample, a method of separating by locally irradiating the interfacebetween the sapphire substrate and the GaN thick film with a laser lightbeam and subliming the interface (a laser lift off method).

However, according to this method, only a small part of the interface isseparated because an area irradiated is small, and stress concentrateson a small attaching part, with the result that cracks are caused.Moreover, since the area irradiated is small, a time period fortreatment is long.

By the way, as a method for reducing dislocations caused in the nitridesemiconductor thick film, and ELO growth (Epitaxial Lateral Overgrowth)method is proposed.

The conventional ELO growth method is exemplified in FIG. 19.

After an AlN buffer layer 321 is grown on a sapphire substrate 320 asshown in FIG. 19A, a first GaN layer 322 is grown as shown in FIG. 19B.

After that, a SiO₂ film 325 is formed on the first GaN layer 322 asshown in FIG. 19C, and a slit line of SiO₂ is formed in the [11-20]direction in mask treatment as shown in FIG. 19D.

Then, a second GaN layer 323 is grown again as shown in FIG. 19E.

Finally, the sapphire substrate 320 is separated as shown in FIG. 19F.The second GaN layer 323 grows from a slit window, and completely fillsin the SiO₂ line to become a flat film, because a speed of alongitudinal growth in the [1-100] direction is faster than a speed ofgrowth in the [0001] direction. Although dislocation curves in alongitudinal direction in accordance with the longitudinal growth, andpenetration dislocations on the SiO₂ line can be reduced, thepenetration dislocations concentrate on a part of the SiO₂ slit window.Therefore, in order to produce a device by selecting a region with smallpenetration dislocation, it is only necessary to carry out masktreatment on the SiO₂ line.

However, since SiO₂ is filled in, it is difficult to carry out masktreatment on SiO₂. Moreover, since curved dislocations concentrate on acentral portion on the SiO₂ line, there arises a problem of inclinationof crystal orientation in a horizontal direction of the substrate, forexample. Furthermore, since crystal growth is carried out while SiO₂ iscontained, diffusion of Si and oxygen atoms occurs. In addition, the ELOgrowth method needs a complicated production process, and therefore,brings about cost increase.

As described above, according to the conventional production methods,when the substrate made of a material of different kind from nitridesemiconductor and the nitride semiconductor thick film are separated,stress resulting from differences in lattice constants and thecoefficient of thermal expansion causes a warp and cracks on theproduced nitride semiconductor apparatus. Moreover, the productionprocess is complicated in the ELO growth that reduces dislocations, andit is difficult to keep away from a portion on the which penetrationdislocation density concentrates, and to carry out the mask treatment oncontained SiO₂. Furthermore, there arises a problem of inclination ofcrystal orientation in a horizontal direction of the substrate becauseof curved dislocation, for example.

SUMMARY OF THE INVENTION

Accordingly, the present invention was made in consideration of theaforementioned problems, and an object thereof is to provide excellentsemiconductor apparatus and production methods with a small latticedefect by which a good characteristic can be expected.

Another object of the invention is to provide a semiconductor apparatusand production methods which, for example, at a time of growing thenitride semiconductor on the (0001) plane of a ZrB₂ single crystalsubstrate, makes electric resistance from the nitride semiconductor tothe substrate to be small and increases crystalline quality of thenitride semiconductor grown.

Still another object of the invention is to provide a semiconductorapparatus and production methods to reduce unevenness appearing onsurface shape thereof in the case of growing a nitride semiconductor onthe (0001) plane of the diboride single crystal substrate by using anAlN buffer layer, it is desired to reduce unevenness appearing on asurface shape thereof.

Still another object of the invention thereof is to, in a nitridesemiconductor apparatus, avoid a warp and cracks of the apparatus anddecrease dislocation density, and to provide a nitride semiconductorapparatus and production methods which are uniform.

The invention provides a semiconductor apparatus comprising:

a substrate made of a diboride single crystal expressed by a chemicalformula XB₂, in which X includes at least one of Ti, Zr, Nb and Hf;

a semiconductor buffer layer formed on a principal surface of thesubstrate and made of Al_(y)Ga_(1-y)N (0<y≦1); and

a nitride semiconductor layer formed on the semiconductor buffer layer,including at least one kind or plural kinds selected from among 13 groupelements and As.

Further, the invention provides a semiconductor apparatus comprising:

a substrate made of a diboride single crystal expressed by a chemicalformula XB₂, in which X includes at least one of Ti, Zr, Nb and Hf;

a semiconductor buffer layer formed on a principal surface of thesubstrate and made of (AlN)_(x)(GaN)_(1-x) (0<x≦1); and

a nitride semiconductor layer formed on the semiconductor buffer layer,including at least one kind or plural kinds selected from among 13 groupelements and As.

0.1≦x≦1.0 is preferable.

Consequently, diffusion of B, which is a main element contained in adiboride single crystal substrate, and formation of a nitridesemiconductor containing B on an interface between the substrate and anitride semiconductor are avoided, and it is possible to obtain anitride semiconductor with a small crystal defect, which is good qualityand excellent.

In the invention, the substrate if of ZrB₂ or TiB₂.

In the invention, the substrate is a solid solution containing one or aplurality of impurity elements of 5 atom % or less, the one or aplurality of impurity elements being selected from a group consisting ofTi, Cr, Hf, V, Ta and Nb when the substrate is of ZrB₂, or selected froma group consisting of Zr, Cr, Hf, V, Ta and Nb when the substrate is ofTiB₂.

In the invention, the thickness of the semiconductor buffer layer madeof (AlN)_(x)(GaN)_(1-x) is within a range of 10 to 100 nm.

In the invention, x of the semiconductor buffer layer made of(AlN)_(x)(GaN)_(1-x) is 0.1≦x≦1.

In the invention, x of the semiconductor buffer layer made of(AlN)_(x)(GaN)_(1-x) is 0.4≦x≦0.6.

In the invention, an angle θ1 formed by a normal line of the principalsurface of the substrate and a normal line of the (0001) plane of thesubstrate is 0°≦θ1≦5°.

Further, the invention provides a method for growing a nitridesemiconductor, comprising:

on a substrate of a diboride single crystal expressed by a chemicalformula XB₂, in which X includes at least one of Ti, Zr, Nb and Hf,growing Al_(y)Ga_(1-y)N layer (0<y≦1) from vapor phase, andsubsequently, growing a nitride semiconductor layer including at leastone kind selected from among 13 group elements and As from vapor phase.

Further, the invention provides a method for growing a nitridesemiconductor, comprising:

on a substrate of a diboride single crystal expressed by a chemicalformula XB₂, in which X includes at least one of Ti, Zr, Nb and Hf,growing an (AlN)_(x)(GaN_(1-x) layer (0<x≦1) from vapor phase within atemperature range of more than 400° C. and less than 1100° C. by anMOVPE method, and subsequently, growing a nitride semiconductor layerincluding at least one kind selected from among 13 group elements and Asfrom vapor phase.

In the invention, the thickness of the (AlN)_(x)(GaN)_(1-x) layer iswithin a range of 10 to 100 nm.

According to the invention, as in the aforementioned structure, as aresult of growing an (AlN)_(x)(GaN)_(1-x) layer (0<x≦1) from vapor phaseon the diboride single crystal substrate by an MOVPE method, andpreferably, defining the thickness of this layer within a range of 10 nmto 100 nm, resistivity of (AlN)_(x)(GaN)_(1-x) becomes lower than thatof AlN, with the result that resistance from the nitride semiconductorto the substrate is decreased, and quality is increased because ana-axis lattice constant of (AlN)_(x)(GaN)_(1-x) is closer to an a-axislattice constant of the (0001) plane of the ZrB₂ single crystal thanAlN. 0.1≦x≦1.0 is preferable. When x is less than 0.1, an(AlN)_(x)(GaN)_(1-x) layer formed on the substrate made of XB₂ becomesto be exfoliated easily.

Further, it is possible to decrease resistance of (AlN)_(x)(GaN)_(1-x)by doping, with the result that it is possible to reduce resistance fromthe nitride semiconductor to the substrate.

As described above, according to the invention, in the case of growing anitride semiconductor layer which contains at least one selected fromamong B, Al, Ga, In and Ti on the (0001) plane of the diboride singlecrystal substrate expressed by a chemical formula XB₂, in which Xcontains at least one of Ti, Zr, Nb and Hf, by the MOVPE method, bygrowing the (AlN)_(x)(GaN)_(1-x) layer (0<x≦1) between the diboridesingle crystal substrate and the nitride semiconductor layer at 400° C.to 1100° C., and preferably, growing to film thickness of 10 nm to 100nm, crystalline quality of the nitride semiconductor is increased asapparent from that a rocking curve half value width of (0002) planeomega scan by an x-ray diffraction method is a value smaller than 1000seconds. Therefore, according to the invention, a characteristic and theyield of a device such as a light emitting diode produced on the ZrB₂single crystal substrate are increased.

Further, according to the invention, serial resistance is decreasedbecause resistivity of (AlN)_(x)(GaN)_(1-x) is lower than that of AlN.Furthermore, it is possible to decrease resistance of(AlN)_(x)(GaN)_(1-x) by doping, so that it is possible to reduceresistance from the nitride semiconductor to the substrate.Consequently, it is possible to decrease a driving voltage of a lightemitting diode of type shown in FIG. 3, which makes it possible toobtain a large number from one wafer.

In the invention, the semiconductor buffer layer is AlN.

In the invention, the thickness of the semiconductor buffer layer madeof AlN is 10 to 250 nm.

In the invention, an angle θ1 formed by a normal line of the principalsurface of the substrate and a normal line of the (0001) plane of thesubstrate is 0°≦θ1≦0.55°.

Further, the invention provides a method for growing a nitridesemiconductor, comprising:

on the (0001) plane of a substrate of a diboride single crystalexpressed by a chemical formula XB₂, in which X includes at least one ofTi, Zr, Nb and Hf, growing an AlN layer from vapor phase so that adeviation angle of a normal line of a surface of the substrate from adirection of the [0001] becomes 0.55 degrees or less, and subsequently,growing a nitride semiconductor layer including at least one kindselected from among 13 group elements and As from vapor phase.

In the invention, the thickness of the AlN layer is within a range of 10to 250 nm.

As described above, according to the invention, in the case of growingthe nitride semiconductor layer containing at least one selected fromamong B, Al, Ga, In and Ti on the (0001) plane of the diboride singlecrystal substrate expressed by the chemical formula XB₂, in which Xcontains at least one of Ti, Zr, Nb and Hf, by the MOVPE method, byusing a method of growing an AlN layer of 10 nm to 100 nm on the (0001)plane of the diboride single crystal substrate at 800° C. or less andthereafter growing the nitride semiconductor layer containing at leastone selected from among B, Al, Ga, In and Ti, wherein a substrate inwhich a deviation angle of a normal line on a surface of the diboridesingle crystal substrate from a [0001] direction of diboride crystal is0.55° degrees or less is used, it is possible to grow a nitridesemiconductor layer which has a smooth surface.

As mentioned above, in a constitution that the buffer layer is made ofAlN, the deviation angle θ1 is selected less than 0.55°. Thereby, it ispossible to prevent an undesirable scaly asperity from being formed on asurface of the nitride semiconductor layer formed on the buffer layer.

The thickness of the semiconductor buffer layer made of AlN is selectedin a range of 10 nm to 250 nm. In the case of less than 10 nm, theeffect of the semiconductor buffer layer is insufficient and the scalyasperity exists on the surface of the nitride semiconductor layer formedon the semiconductor buffer layer. In the case of larger than 250 nm,film quality of the nitride semiconductor layer formed on thesemiconductor buffer layer becomes bad, and a crystal layer as thenitride semiconductor layer formed on the semiconductor buffer layer,which crystal layer is formed above a crystal plane of the substrate viathe semiconductor buffer layer, degrades.

Therefore, by the use of the invention, a device such as a lightemitting diode produced on the ZrB₂ single crystal substrate can beproduced on a smooth plane, and a characteristic and the yield thereofare increased. The thickness of the semiconductor buffer layer is soselected as to become thinner than the thickness of a crystal layer madeof nitride semiconductor formed on the semiconductor buffer layer first.

In the invention, the substrate is eroded and removed by etching.

Further, the invention provides a method for producing a semiconductorapparatus, comprising:

eroding and removing a diboride single crystal substrate of asemiconductor apparatus obtained by the aforementioned method forgrowing nitride semiconductors by etching.

Further, the invention provides a method for producing a semiconductorapparatus, comprising the steps of:

carrying out crystal growth of a nitride semiconductor layer on oneprincipal surface of a single crystal substrate of a hexagonal crystalsymmetry having electrical conductivity; and

eroding and removing the single crystal substrate by etching.

In the invention, the single crystal substrate is a substrate of adiboride single crystal expressed by XB₂, in which X includes at leastone of Zr and Ti.

In the invention, in growing the nitride semiconductor layer from vaporphase, a nitride semiconductor layer grown firstly is an Al_(x)Ga_(1-x)Nlayer (0<x≦1).

In the invention, a mixed solution of at least nitric acid andhydrofluoric acid is used for the etching.

As described above, according to a method for producing a semiconductorapparatus of the invention, by going through the step of carrying outcrystal growth of a nitride semiconductor layer on one principal surfaceof a single crystal substrate of a hexagonal symmetry structure havingelectrical conductivity, and the step of eroding and removing the singlecrystal substrate by etching, it is possible to avoid a warp and cracksof the semiconductor apparatus, decrease dislocation density, curvedislocation, prevent inclination of crystal orientation in a horizontaldirection of the semiconductor apparatus, and thereby provide a nitridesemiconductor apparatus in which in-plane uniformity of dislocationdensity is good.

BRIEF DESCRIPTION OF THE DRAWINGS

Other and further objects, features, and advantages of the inventionwill be more explicit from the following detailed description taken withreference to the drawings wherein:

FIG. 1 is a sectional view schematically explaining a semiconductorapparatus of the invention;

FIG. 2 is a sectional view showing a diboride single crystal substrate(chemical formula XB₂) of the semiconductor apparatus shown in FIG. 1;

FIGS. 3A and 3B are crystal structural views of XB₂ respectively;

FIGS. 4A and 4B are crystal structural views of XB₂ respectively;

FIG. 5 is a view showing a surface state B of GaN film;

FIG. 6 is a view showing a surface state A of GaN film;

FIG. 7 is a graph showing a relation between an angle formed by a normalline of a principal surface of the substrate and a normal line of the(0001) plane and a surface state;

FIG. 8 is a schematically sectional view of the semiconductor apparatusin which a nitride semiconductor layer is formed on a ZrB₂ singlecrystal substrate;

FIG. 9 is a view showing of photographs of surfaces of GaN films;

FIG. 10 is a line view showing a relation between film thickness of(AlN)_(x)(GaN)_(1-x) and X-ray half value width;

FIG. 11 is a schematically sectional view of the semiconductor apparatusin which a nitride semiconductor layer is formed on a ZrB₂ singlecrystal substrate;

FIG. 12 is a line view showing a relation between an off angle of asubstrate surface and surface state;

FIG. 13 is a view showing a surface state (surface state B) of GaN film;

FIG. 14 is a view showing a surface state (surface state A) of GaN filmaccording to the invention;

FIGS. 15A thorough 15F are process views showing a method for producinga semiconductor substrate of the invention;

FIG. 16 is a crystal structural view of diaboride single crystaldescribed as ZrB₂;

FIG. 17 is a sectional view explaining a conventional semiconductorapparatus;

FIG. 18 is a schematically sectional view of the semiconductor apparatusin which a nitride semiconductor layer is formed on a sapphiresubstrate; and

FIGS. 19A through 19F are process views showing a method for producing aconventional semiconductor substrate.

DETAILED DESCRIPTION

Now referring to the drawings, preferred embodiments of the inventionare described below.

FIG. 1 is a sectional view showing a semiconductor apparatus accordingto one embodiment of the invention. FIG. 2 is a sectional view showing adiboride single crystal substrate (chemical formula XB₂) 10 ofsemiconductor apparatus shown in FIG. 1. A normal line of a principalsurface 34 of the substrate 10 is inclined with respect to a crystalaxis 32 which is a [0001] axis perpendicular to a (0001) plane 31 of thesubstrate 10, by an angle θ1, which is larger than or equal to 0° andless than or equal to 5° (0°≦θ1≦5°). That is, it is preferable that thediboride single crystal substrate (chemical formula XB₂) 10 of thepresent invention is the substrate 10 such that the (0001) plane 31 or aplane obtained by inclining the (0001) plane 31 by 0° or more and 5° orless in an arbitrary direction is defined as the principal surface 34.In order to make crystalline quality of a nitride semiconductor layer 11through 16 grown on the substrate 10 to be fine, and obtain asemiconductor apparatus which has a more excellent characteristic, anangle θ1 formed by a normal line 33 of a principal surface 34 of thesubstrate and a normal line 32 of the (0001) plane 31 is set to 0° ormore and less than 1.7° (0°≦θ1≦1.7°). Preferably, the angle is set to 0°or more and less than 0.7° (0°≦θ1<0.7°). Further, a (01-10) plane, a(11-20) plane, a (01-12) plane and so on, other than the (0001) plane,can also be used as the growth principal plane. Symbol ‘−’ of ‘−1’ and‘−2’ by Miller index expression represents an inverse (bar) symbol, andthe same applies in the following description.

In specific, TiB₂ and ZrB₂ wherein X is Ti and Zr have differences inlattice constant of 2% or less in any composition of the nitridesemiconductor made of Al, Ga and N which are elements constituting thesemiconductor buffer layer. For example, in a semiconductor apparatus inwhich the nitride semiconductor is AlN and the substrate is constitutedby ZrB₂, referring to Table 1 mentioned above, the difference in latticeconstant is (3.1696−3.1114)/3.1114=1.9%. Thus, they become a combinationof extremely high matching. The lattice constant of the nitridesemiconductor containing Al or Ga and N, namely, Al_(y)Ga_(1-y)N(0<y≦1), is in a range of 3.1114 to 3.1860 with reference to Table 1mentioned above. According to the invention, it is enough that at leastone of the elements of Ti and Zr on diaboride single crystal substrateis contained, and both the elements of Ti and Zr may be contained.

A crystal structure of XB₂ is a hexagonal symmetry structure referred toas an AlB₂ structure as shown in FIGS. 3A, 3B. This structure is similarto a wurtzite structure of a GaN crystal shown in FIGS. 4A, 4B. Inspecific, regarding a match relation of crystal lattice between the(0001) plane of an XB₂ crystal of Ti or Zr and GaN or AlN, as shown inTable 1, a difference in lattice constant between TiB₂ or ZrB₂ and GaNor AlN is 2% or less in any combination, and therefore, they can beconsidered as a combination of extremely high matching.

For forming semiconductor buffer layer in crystal growth, a molecularbeam epitaxy (MBE) method, a metalorganic vapor phase epitaxy (MOCVD)method, a hydride vapor epitaxy (HVPE) method, a sublimation method andthe like are used. Moreover, it is possible to appropriately combine theaforementioned growing methods as well. For example, it is possible touse the MBE method, by which it is possible to grow while controlling asurface state, for initial epitaxy growth, and use the HVPE method, bywhich it is possible to grow at high speeds, for a thick GaN thin filmrequired.

Next, after a buffer layer 11 is formed, a nitride semiconductor whichcontains a 13 group (former IIIB group) element to be aimed is formed.Here, crystal growth of the nitride semiconductor is carried out atgrowth temperatures of 700° C. to 900° C. At his moment, B, which is amain element contained in the diboride single crystal substrate 10,diffuses from a substrate side into the nitride semiconductor of thebuffer layer 11.

In the invention, a semiconductor buffer layer composed of at leastAlGaN is used as the buffer layer 11. In the nitride semiconductor, aninteratomic distance of AlN is smaller than interatomic distances of InNand GaN. Therefore, crystal bond in AlN is stronger than those in InNand GaN, and diffusion of B from the diboride single crystal substratein AlN is less than in InN and GaN.

Further, as shown in aforementioned Table 1, InN and InGaN largelymismatch with the diboride single crystal substrate in latticeconstants, and therefore, in the case of using InN and InGaN as thebuffer layers and carrying out crystal growth directly on the substrate,a lattice defect or the like occurs. On the contrary, in the invention,it is preferable since AlGaN matches with the diboride single crystalsubstrate in lattice constants.

Further, in specific, the nitride semiconductor containing the 13 groupelement contains one or more selected from among Ga, Al, In, H and Tiand moreover may contain As which is the 15 group element.

Regarding the diboride single crystal, assume one or more kinds ofimpurity elements selected from among Cr, Hf, V, Ta and Nb, which are 4group to 6 group (former IVA group to VIA group) elements, are solidsolutions of 5 atom % or less. This is because when the impurityelements are more than 5 atom %, a value of physical property shown inTable 1 and a value of specific resistance of the substrate vary, whichis not preferable. It is possible by containing Cr of 5 atom % or lessto expect an effect of inhibiting growth of crystal grains of thenitride semiconductor layer, and therefore, it is preferable for forminga good layer without occurrence of cracks or the like.

In this way, according to the invention, it is possible to avoiddiffusion of B, which is a main element contained in the diboride singlecrystal substrate, and formation of a nitride semiconductor containing Bon an interface between the substrate and the nitride semiconductor, andit is possible to obtain a good quality nitride semiconductor with asmall lattice defect, and thus, it is possible to obtain a semiconductordevice which has an excellent characteristic.

Besides, a nitride semiconductor apparatus (a light emitting diode)which contains the 13 group element shown in FIG. 1 will be described asan embodiment of the invention.

A GaN layer is grown on the (0001) plane of a substrate 10 of ZrB₂ bythe molecular beam epitaxy (MBE) method. On a ZrB₂ single crystalsubstrate 10 of the (0001) plane orientation, AlGaN of a buffer layer11, which is a semiconductor buffer layer, and crystal growth of nitridesemiconductor 12 through 16 to be aimed is carried out on the bufferlayer 11 by the MBE method in order. In high vacuum, a temperature ofthe ZrB₂ substrate 10 is increased to 800° C. and an Al molecular beam,a Ga molecular beam and active nitrogen supplied from a high frequencyexcitation plasma cell are supplied to start crystal growth.

Here, one conductive type, which is one of p type and n type, ofsemiconductor contact layer 12 on the buffer layer 11 is made of n-GaN,for example. The one conductive type of semiconductor contact layer 12contains approximately 1×10¹⁷ to 10¹⁹ atoms/cm³ of one conductive typeof semiconductor impurity, such as silicon. Furthermore, a oneconductive type of semiconductor layer 13 on the layer 12 is a claddinglayer made of n-AlGaN, for example. Moreover, a one conductive type ofsemiconductor layer 13 contains approximately 1×10¹⁶ to 10¹⁹ atoms/cm³of one conductive type of semiconductor impurity, such as silicon.

A light emitting layer 14 is formed on the layer 13 and made of GaN,InGaN or the like. Here, the light emitting layer 14 may have a quantumwell structure, a quantum thin line structure of a quantum dotstructure.

A reverse conductive type, which is the other of p type and n type, ofsemiconductor layer 15 is formed on the layer 14 and is a cladding layermade of p-AlGaN or the like, and contains approximately 1×10¹⁶ to 10¹⁹atoms/cm³ of impurity which changes into a reverse conductive type, suchas Mg or Zn. Here, this layer may contain a small amount of one or moreselected from among In, P, As and the like.

A reverse conductive type of semiconductor contact layer 16 is formed onthe layer 15 is made of ZrB₂, for example, and contains approximately1×10¹⁹ to 10²⁰ atoms/cm³ of impurity which changes into a reverseconductive type, such as Mg or Zn. A portion from the layer 16 to anupper region of the layer 12 is partly etched and removed.

Thereafter, one conductive type of electrode 18 on the layer 12 is madeof one or more selected from among Au, Al, Cr, Ti and Ni. Moreover, areverse conductive type of electrode 17 on the layer 16 is made of oneor more selected from among Au, Al, Cr, Ti and Ni as well.

In this way, this embodiment also realizes an excellent semiconductorapparatus with a small lattice defect, by which a good characteristiccan be expected. Here, a layer structure of the semiconductor apparatusis not restricted to the structure shown in FIG. 1, and it may be astructure such that nitride semiconductor layers 112 to 117 are formedon one principal surface of a substrate 118 such as FIG. 8described-after, one electrode 111 is formed on the nitridesemiconductor layer 112 and another electrode 119 is formed on anotherprincipal surface of the substrate 118.

Next, in the respective embodiments in FIGS. 1 to 4, the result of astudy of the principal surface 34 of the substrate 10 for preferablygrowing the nitride semiconductor layer and a most suitable crystalplane 31 will be described.

Step 1A:

Firstly, several kinds of ZrB₂ single crystal substrates 10 which havedifferent off angles (angles θ1 formed by the normal line 33 of theprincipal surface 34 of the substrate 10 and the normal line 32 of the(0001) plane 31) were prepared. The surface of ZrB₂ was cleansed by theuse of an alkali solvent.

Step 2A:

Before the nitride semiconductor was grown, the substrate 10 was heatedfor three minutes in a hydrogen (H₂) atmosphere (1 air pressure), andannealed at 1150° C. for one minute.

Step 3A:

After that, temperature was decreased for five minutes, and an AlGaNlayer serving as a semiconductor buffer layer 11 was grown. A growthtemperature was 850° C. and film thickness was 20 nm at that moment.Further, ammonia (NH₃), trimethylaluminum (TMAI) and trimethylgallium(TMGa) were used as source gases, the amounts of supplied NH₃, TMAI andTMGa were 0.07 mol/min, 8 μmol/min and 11 μmol/min, respectively, and 7slm of H₂ was flown as a carrier gas. NH₃ was supplied from one minutebefore supply of TMA.

Step 4A:

Next, the temperature was increased to 1150° C., and GaN serving as thenitride semiconductor layer 12 was grown to thickness of approximately 3μm. NH₃ and TMGa were used as source gases, and the amount of suppliedTMGa and NH₃ were 44 μmol/min and 0.07 mol/min, respectively. Moreover,3 slm of H₂ was flown as a carrier gas.

In microscopic observation of surfaces of the GaN films 12 after growth,surfaces with much unevenness as shown in FIG. 5 (surface state B) andsurfaces of smooth states so shown in FIG. 6 (surface state A) wereobserved, respectively.

A relation between the off angles of the ZrB₂ single crystal substrates10 and the surface states of the grown films is shown in FIG. 7. Here,an angle O2 of deviation from a [0001] crystal axis 32 of the normalline 33 of the surface 34 of the substrate 10 in the [10 10] direction,an angle O3 of deviation of the same in the [11-20] direction, and thesum of squares of these deviation angles (−O2 ²+θ3 ²) are shown by thelines 36, 37 and 38, respectively. All substrates that the sums ofsquares of the deviation angles were less than 0.7° kept good surfacestates of the surface state A.

Regarding substrates that the sums of squares of the deviation angleswere 0.7° or more and less than 1.7°, both the surface state A and thesurface state B were observed. It can be concluded that this resultsfrom variation of operations in the growth experiment and states of thedevice, and it is believed that the surface state A can be reproduced byreducing the variation. In the case of substrates that the sums ofsquares of the deviation angles were 1.7° or more, almost all of thesubstrates kept the surface state B.

From these results, it became clear that in order to grow a nitridesemiconductor layer containing one or more selected from among 13 groupelements in a suitable crystal state and thus obtain a semiconductorapparatus which is excellent in a characteristic such as the efficiencyof light emission, it is more desirable to make an angle θ1 formed bythe normal line 33 of the principal surface 34 of the substrate 10 andthe normal line 32 of the (0001) plane 31 to be 0° or more and less than1.7°, and that in the case of forming a good quality nitridesemiconductor layer, a crystal angle of the principal surface of thesubstrate has the aforementioned allowance range.

Although (1) a GaN growth layer is formed by the use of a ZrH₂ substratein the aforementioned embodiment, it is also possible to form a nitridesemiconductor layer containing a 13 group element, such as a GaN growthlayer, on (2) a single crystal substrate made of TiB₂ or (3) a singlecrystal substrate formed of solid solutions of ZrB₂ and TiB₂ in the samemanner as described above, and it is possible to appropriately changeand embody the invention within the scope of the invention.

The steps in the invention will be sequentially described below.

FIG. 8 is a sectional view showing a semiconductor apparatus of a lightemitting diode according to another embodiment of the invention.

According to the invention, a diboride single crystal substrate 118expressed by a chemical formula XB₂ is used, in which X contains atleast one of Ti, Zr, Nb and Hf.

This substrate 118 can be to a ZrB₂ substrate, a TiB₂ substrate or aZr_(x)Ti_(1-x)B₂ substrate, however, a method for carrying out vaporphase growth on the ZrB₂ substrate by the MOVPE method will be describedin this embodiment.

Step 1B:

The ZrB₂ substrate 118 is cleansed by the use of an alkali solvent.

Step 2B:

Before growth of a nitride semiconductor, the ZrB₂ substrate 118 isheated for three minutes in a hydrogen (H₂) atmosphere (1 air pressure),and annealed at a temperature of 1150° C. for one minute.

Step 3B:

After that, temperature was decreased for about five minutes, and an(AlN)_(x)(GaN)_(1-x) buffer layer 117 is deposited.

It is good to set a growth temperature T within a temperature range of400° C.<T<1100° C. at this moment, and then grow the(AlN)_(x)(GaN)_(1-x) layer (0<x≦1.0) from vapor phase as the bufferlayer 117.

Further, it is good to set thickness of the (AlN)_(x)(GaN)_(1-x) layer117 within a range of 10 to 100 nm. According to such vapor phasegrowth, used source gases are ammonia (NH₂), trimethylaluminum (TMA) andtrimethylgallium (TMG), the amounts of supplied TMA and TMG are, forexample, 7 μmol/min and 11 μmol/min, respectively, and 7 slm of H₂ isflown as a carrier gas. 0.07 mol/min of NH₃ is supplied from one minutebefore supply of TMA and TMG.

Step 4B:

Next, the substrate is heated to 1150° C., for example, and an n-GaNcontact layer 116 is grown to approximately 3 μm by the MOVPE method,for example. Used source gases are NH₃ and TMG, and the amounts ofsupplied TMG and NH₃ are, for example, 44 μmol/min and 0.07 mol/min,respectively. 3 slm of H₂ is flown a carrier gas.

Further, a cladding layer 115 made of n-AlGaN, a light emitting layer134 made of GaN, InGaN or the like, a cladding layer 113 made ofp-AlGaN, a contact layer 112 made of p-GaN and a p-electrode 111 areformed, and an n-electrode 119 is formed on a back surface of thesubstrate 118.

In the case of using the ZrB₂ single crystal substrate 118, since such astructure that one electrode 119 is arranged on the back surface of thesubstrate 110 is possible, there is an advantage that device area can bereduced.

Next, an experimental example which was carried out by the inventorswill be described.

Although the aforementioned steps 1B to 4B were executed sequentially,according to the example, a deposition temperature was changed among400° C., 725° C., 850° C. and 1100° C. and film thickness was changedamong 10 nm, 20 nm, 50 nm and 80 nm at step 3B, whereby various kinds ofsamples were produced.

Source gases were ammonia (NH₃), trimethylaluminum (TMA) andtrimethylgallium (TMG), the supply amounts of TMA and TMG were 7μmol/min and 11 μmol/min, respectively, and 7 slm of H₂ was flown as acarrier gas. 0.07 mol/min of NH₃ was supplied from one minute beforesupply of TMA and TMG. Next, at step 4B, the substrate was heated to1150° C. and GaN was grown to approximately 3 μm. Used source gases wereNH₃ and TMG, and the amounts of supplied TMG and NH₃ were 44 μmol/minand 0.07 mol/min, respectively. 3 slm of H₂ was flown as a carrier gas.

The results of the experiment on the various kinds of samples obtainedby changing the deposition temperature and the film thickness at step 3Bas described above are shown in FIG. 9.

FIG. 9 shows photographs of surfaces of GaN films in the case of growingthe (AlN)_(x)(GaN)_(1-x) layer 117 at temperatures of 400° C. or less,850° C., and 1100° C. or more.

Further, a value of x, and a rocking curve half value width of (0002)plane omega scan by an X-ray diffraction method are shown on the rightof description of the temperature given in each of the FIG. 9 (refer toTable 2).

The value of x is data obtained by measuring the (AlN)_(x)(GaN)_(1-x)films grown under the same condition, by an EDX method. TABLE 2 HalfDeposition value Working example/ temperature T width Comparativeexample (° C.) x (seconds) (1) Comparative example 1 400 or less 0.8 —(2) Working example 1 725 0.6 760 (3) Working example 2 850 0.5 596 (4)Working example 3 925 0.4 — (5) Comparative example 2 1100 or more 0.25873

As apparent from these results, the GaN film was not formed at 400° C.or less, and the GaN film had a hexagonal surface shape at 1100° C. ormore. The GaN films had smooth surfaces at 850° and 725° C. Here, therocking curve half value width was 1000 seconds or less in every casethat the growth temperature T was more than 400° C. and less than 1100°C.

A relation between film thickness of the (AlN)_(x)(GaN)_(1-x) layerdeposited at 850° C. and the rocking curve half value width of the(0002) plane omega scan by the X-ray diffraction method is shown in FIG.10. In a case where the layer was grown in a setting such that the filmthickness was less than 10 nm, a surface state thereof became the sameas that at deposition temperatures 400° C. or less. It can be read fromthe graph that the half value width becomes 1000 seconds or less betweenthe film thickness 10 nm and 100 nm.

Further, for comparison, in a comparative example 3, after the AlN layerwas deposited on the ZrB₂ substrate at 600° C., GaN was grown toapproximately 3 μm at 1150° C. Source gases used at the time of growingthe AlN layer were NH₃ and TMA. The amounts of supplied TMA and NH₃ asthe sources were 3.5 μmol/min and 0.07 mol/min, respectively, and 2 slmof H₂ was flown as a carrier gas. Conditions other than the above werethe same as those of the (AlN)_(x)(GaN)_(1-x) layer. The rocking curvehalf value width of the (0002) plane omega scan by X-ray diffraction wasapproximately 1000 seconds.

Further, as a comparative example 4, instead of growing the(AlN)_(x)(GaN)_(1-x) layer (0<x<1.0) from vapor phase, the GaN layer wasgrown at 400° C., and thereafter, according to step 4B, the temperaturewas increased to 1150° C. to grow the GaN layer to approximately 3 μm bythe MOVPE method.

According to such vapor phase growth of the GaN layer at a depositiontemperature 400° C., conditions were the same as those of the AlN layerexcept that the source was replaced from TMA to TMG.

As a result of production in this manner in comparative example 3, theGaN film exfoliated from the ZrB₂ substrate right away after growth.

Here, the invention is not restricted to the above embodiment, and canbe changed and improved in various manners within the scope of theinvention. For example, although the ZrB₂ substrate was used as thediboride single crystal substrate, it was confirmed by an experimentthat, in place of the above substrate, a substrate such that a chemicalformula is XB₂ and X is Ti, Nb or Hf or a combination thereof also takesthe operations and effects of the invention.

According to the invention, a diboride single crystal substrate 218expressed by a chemical formula XB₂ is used, in which X contains atleast one of Ti, Zr, Nb and Hf.

This substrate 218 can be to a ZrB₂ substrate, a TiB₂ substrate or aZr_(x)Ti_(1-x)B₂ substrate, however, a method for carrying out vaporphase growth on the ZrB₂ substrate 218 by the MOVPE method will bedescribed in this embodiment.

Step 1C:

The ZrB₂ substrate 218 is cleansed by the use of an alkali solvent.

Step 2C:

Before growth of a nitride semiconductor layer 217, the ZrB₂ substrate218 is heated for three minutes in a hydrogen (H₂) atmosphere (1 airpressure), and annealed at a temperature of 1150° C. for one minute.

Step 3C:

After that, temperature is decrease for about five minutes, and an AlNlayer 217 which is low temperature buffer layer is deposited.

At this moment, it is good to set a growth temperature T within atemperature range of 800° C. or less, and grow the AlN layer 217 fromvapor phase. Here, in the present embodiment, the temperature is set to600° C. Further, it is good to set the thickness of the AlN layer 217within a range of 10 nm to 250 nm. Here, in the embodiment, thethickness is set to 20 nm.

According to such vapor phase growth, ammonia (NH₃), trimethylaluminum(TMA) and trimethylgallium (TMG) are used as source gases, the amountsof supplied NH₃ and TMA are, for example, 0.07 mol/min and 3.5 μmol/min,respectively, and 4 slm of H₂ is flown as a carrier gas. NH₃ is suppliedfrom one minute before supply of TMA.

Step 4C:

Next, the substrate is heated to 1150° C., for example, and a p-GaNcontact layer 216 is grown to approximately 3 μm by the MOVPE method,for example. Used source gases are NH₃ and TMG, and the amounts ofsupplied TMG and NH₃ are, for example, 44 μmol/min and 0.07 mol/min,respectively. 3 slm of H₂ is flown a carrier gas.

A cladding layer 215 made of n-GaN, a light emitting layer 214 made ofInN, InGaN or the like, a cladding layer 213 made of p-GaN, a contactlayer 312 made of p-GaN are formed on the p-type contact layer 216 inthis order, and a p electrode 211 are further formed. An n electrode 219is formed on a back surface of the substrate 218.

In observation of surfaces of the GaN films 216 after growth, an unevensurface as shown in FIG. 13 (surface state H) and a smooth surface asshown in FIG. 14 (surface state A) are observed.

Then, a relation between an off angle θ1 of the ZrB₂ single crystalsubstrate 218 and a surface state of the grown film is shown in FIG. 12.Here, an angle θ4 of deviation of a normal line 33 (refer toaforementioned FIG. 12) on a surface of a substrate 218 from a [0001]crystal axis 32 in the [10-10] direction, an angle θ5 of deviation ofthe same in the [11-20] direction, and the sum of squares (−θ4 ²|O5 ²)thereof are shown by lines 236, 237 and 238 in FIG. 12, respectively.All substrates that the sums of squares of the off angle are 0.35degrees or less keep the surface state A. In the case of substrates thatthe sum of squares of the off angles are between 0.35 degree and 0.55degrees, both the surface state A and the surface state B are observed.It can be concluded that this results from variation of operations inthe growth experiment and states of the device, and it is believed thatthe surface state A can be reproduced by reducing the variation. Allsubstrates that the sums of squares of the off angles are 0.55 degreesor more keep the surface state B.

Here, the invention is not restricted to the above embodiment, and canbe changed and improved in various manners within the scope of theinvention. For example, although the ZrB₂ substrate was used as thediboride single crystal substrate 218, it was confirmed by an experimentthat, in place of the above substrate, a substrate such that a chemicalformula is XB₂ and X is Ti, Nb or Hf or a combination thereof also takesthe operations and effects of the invention.

FIG. 15 is a process view showing a method for producing a nitridesemiconductor device according to one embodiment of the invention, andFIG. 16 shows a crystal structure of a single crystal substrate 310which has a hexagonal crystal symmetry.

Firstly, the step of crystal-growing a nitride semiconductor layer 313on one principal surface of the single crystal substrate 310 will bedescribed.

The substrate 310 used in the invention is a diboride single crystalsubstrate having a metallic or semimetallic characteristic whose crystalstructure is a hexagonal symmetry structure.

Preferably, in the case of using a diboride single crystal substrateexpressed by, for example, ZrB₂ as the single crystal substrate 310 isshown in FIG. 16, it is good to make the (0001) plane to be theprincipal surface of the substrate.

Further, as compared with on insulating material, this single crystalsubstrate 310 has the same level of electric conductivity as semimetalbecause it is a high electric conductor.

Regarding ZrB₂ whose lattice constant a is 3.170 Å, as compared with GaNof wurtzite structure (a lattice constant: a=3.189 Å), a ration of alattice mismatch thereof is 0.60% (−3.189−3.170)/3.170), which is small,and a difference in the coefficient of thermal expansion is 2.7×10⁻⁶/K,and therefore, it achieves an extremely highly matching combination,with the result that it is possible to obtain a good quality nitridesemiconductor such that a lattice defect is small and stress between thesubstrate and the nitride semiconductor is small.

In crystal growth of the buffer layer 311, a molecular beam epitaxy(MBE) method, a metalorganic vapor Phase epitaxy (MOCVD) method, ahydride vapor phase epitaxy (HVPE) method, a sublimation method and thelike are used.

Further, these growth methods may be used in combination. For example,it is possible to use the MBE method or the MOCVD method, by which it ispossible to grow while controlling a surface state, for initial epitaxygrowth, and use the HVPE method, by which it is possible to grow at highspeeds, for a thick GaN thin film 311 required.

Firstly, a buffer layer 311 is formed, and then, a nitride semiconductor312 and 313 of required layer structure are formed. The buffer layer 311and the nitride semiconductor 312 and 313 are grown from vapor phase atgrowth temperatures of 600° C. to 1100° C.

According to such crystal growth, the substrate 310 is heated by thermalconduction from a susceptor heated by a heater or the like. Thermalenergy from the susceptor is thermally conducted to the single crystalsubstrate (ZrB₂ substrate) 310 by thermal contact. Moreover, thermalenergy is emitted as thermal radiation (infrared radiation) from thesurface of the susceptor, other than by the thermal contact. The singlecrystal substrate 310 made of a metallic or semimetallic substrateabsorbs the thermal radiation, and is heated. Being heated, Zr and Bdiffuse a little into the nitride semiconductor 312 and 313.

Next, the step of eroding and removing the single crystal substrate 310by etching will be described.

By applying resist coat treatment to the whole surface of the grown GaNthick film 313 by the use of a photoresist 314, and etching with anetching solution having a selective etching characteristic together withthe ZrB₂ substrate 310, the ZrB₂ substrate 310 is completely eroded.After that, by exfoliating the photoresist 314 by the use of anexfoliation solution, a semiconductor apparatus made of a GaN thick film311, 312 and 313 are obtained.

Thus, according to the method for producing the semiconductor apparatusof the invention, by going through the steps as mentioned above, it ispossible to produce a nitride semiconductor apparatus with lowdislocation density and without curving dislocation, and it is possibleto obtain a high quality nitride semiconductor without inclining crystalorientation in the horizontal direction of the substrate and withoutproducing a dislocation concentrating point within the plane of thesemiconductor apparatus.

Next, an embodiment of the invention such that a GaN apparatus isobtained by the use of a ZrB₂ single crystal substrate 310 will bedescribed.

In the process shown in FIG. 15, steps A to F are executed sequentially.

Step 1D:

An example of forming and growing a buffer layer 311 made of GaN on asubstrate 310 of the (0001) plane of ZrB₂ by using both the MOCVD methodand the HVPE method is shown.

On the ZrB₂ single crystal substrate 310 of the (0001) planeorientation, a buffer layer 311 is grown from vapor phase by the MOCVDmethod.

As shown in FIG. 15A, the substrate 310 of the (0001) plane of ZrB₂ isset in an MOCVD growth furnace, and the temperature of the ZrB₂substrate 310 is increased to 800° C. in a high vacuum to start crystalgrowth. After that, the temperature is decreased to 600° C. to grow thebuffer layer 311. The buffer layer 311 is made of AlGaN, whose rawmaterials are trimethylaluminum (hereinafter referred to as TMA),trimethylgallium (hereinafter referred to as TMG) and ammonia gas, andgrown to thickness of 0.1 μm.

Step 2D:

As shown in FIG. 15B, the temperature of the substrate 310 is increasedto 1050° C., and a first semiconductor layer 312 is grown by the MOCVDmethod.

The first semiconductor layer 312 is made of GaN, whose raw materialsare TMG and ammonia gas, and grown to thickness of 1 μm.

Step 3D:

The substrate 312 grown as FIG. 15B is taken out and moved into an HVPEfurnace, where a second semiconductor layer 313 is grown as shown inFIG. 15C.

The HVPE method is appropriate for production of a thick film ofthickness of several score μm to several hundred μm, because a growthspeed in this method is higher than that in the MOCVD method. The secondsemiconductor layer 313 is made of GaN, whose raw sources are galliumchloride and ammonia gas, and grown to 300 μm.

Step 4D:

Then, the substrate grown as shown in FIG. 15C is taken out, and asurface of the second GaN layer 313 is spin-coated by a photoresist 314and subjected to baking treatment as shown in FIG. 15D.

Step 5D:

After that, as shown in FIG. 15E, the ZrB₂ substrate 310 is completelyeroded by an etching solution composed of nitric acid and hydrofluoricacid.

The etching solution composed of nitric acid and hydrofluoric acid isexcellent in selectivity, because a ratio of an etching rate of GaN andan etching rate of ZrB₂ is 100 times or more.

Step 6D:

After that, the photoresist 314 is exfoliated by an exfoliation solutionas shown in FIG. 15F, whereby a GaN semiconductor apparatus formed by aGaN thick film 313 can be obtained.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiment are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription and all changes which come within the meaning and the rangeof equivalency of the claims are therefore intended to be embracedtherein.

1. A semiconductor apparatus comprising: a substrate made of a diboride single crystal expressed by a chemical formula XB₂, in which X includes at least one of Ti, Zr, Nb and Hf; a semiconductor buffer layer formed on a principal surface of the substrate and made of Al_(y)Ga_(1-y)N (0<y≦1); and a nitride semiconductor layer formed on the semiconductor buffer layer, including at least one kind or plural kinds selected from among 13 group elements and As.
 2. A semiconductor apparatus comprising: a substrate made of a diboride single crystal expressed by a chemical formula XB₂, in which X includes at least one of Ti, Zr, Nb and Hf; a semiconductor buffer layer formed on a principal surface of the substrate and made of (AlN)_(x)(GaN)_(1-x) (0<x≦1); and a nitride semiconductor layer formed on the semiconductor buffer layer, including at least one kind or plural kinds selected from among 13 group elements and As.
 3. The semiconductor apparatus of claim 1, wherein the substrate is of ZrB₂ or TiB₂.
 4. The semiconductor apparatus of claim 2, wherein the substrate is of ZrB₂ or TiB₂.
 5. The semiconductor apparatus of claim 1, wherein the substrate is a solid solution containing one or a plurality of impurity elements of 5 atom % or less, the one or a plurality of impurity elements being selected from a group consisting of Ti, Cr, Hf, V, Ta and Nb when the substrate is of ZrB₂, or selected from a group consisting of Zr, Cr, Hf, V, Ta and Nb when the substrate is of TiB₂.
 6. The semiconductor apparatus of claim 2, wherein the substrate is a solid solution containing one or a plurality of impurity elements of 5 atom % or less, the one or a plurality of impurity elements being selected from a group consisting of Ti, Cr, Hf, V, Ta and Nb when the substrate is of ZrB₂, or selected from a group consisting of Zr, Cr, Hf, V, Ta and Nb when the substrate is of TiB₂.
 7. The semiconductor apparatus of claim 1, wherein the semiconductor buffer layer is AlN.
 8. The semiconductor apparatus of claim 2, wherein the semiconductor buffer layer is AlN.
 9. The semiconductor apparatus of claim 7, wherein the thickness of the semiconductor buffer layer made of AlN is 10 to 250 nm.
 10. The semiconductor apparatus of claim 8, wherein the thickness of the semiconductor buffer layer made of AlN is 10 to 250 nm.
 11. The semiconductor apparatus of claim 2, wherein the thickness of the semiconductor buffer layer made of (AlN)_(x)(GaN)_(1-x) is within a range of 10 to 100 nm.
 12. The semiconductor apparatus of claim 2, wherein x of the semiconductor buffer layer made of (AlN)_(x)(GaN)_(1-x) is 0.1≦x≦1.
 13. The semiconductor apparatus of claim 2, wherein x of the semiconductor buffer layer made of (AlN)_(x)(GaN)_(1-x) is 0.4≦x≦0.6.
 14. The semiconductor apparatus of claim 1, wherein an angle θ1 formed by a normal line of the principal surface of the substrate and a normal line of the (0001) plane of the substrate is 0°≦θ1≦5°.
 15. The semiconductor apparatus of claim 2, wherein an angle θ1 formed by a normal line of the principal surface of the substrate and a normal line of the (0001) plane of the substrate is 0°≦θ1≦5°.
 16. The semiconductor apparatus of claim 7, wherein an angle θ1 formed by a normal line of the principal surface of the substrate and a normal line of the (0001) plane of the substrate is 0°≦O1≦0.55°.
 17. The semiconductor apparatus of claim 8, wherein an angle O1 formed by a normal line of the principal surface of the substrate and a normal line of the (0001) plane of the substrate is 0°≦θ1≦0.55°.
 18. The semiconductor apparatus of claim 1, wherein the substrate is eroded and removed by etching.
 19. The semiconductor apparatus of claim 2, wherein the substrate is eroded and removed by etching.
 20. A method for growing a nitride semiconductor, comprising: on a substrate of a diboride single crystal expressed by a chemical formula XB₂, in which X includes at least one of Ti, Zr, Nb and Hf, growing Al_(y)Ga_(1-y)N layer (0<y≦1) from vapor phase, and subsequently, growing a nitride semiconductor layer including at least one kind selected from among 13 group elements and As from vapor phase.
 21. A method for growing a nitride semiconductor, comprising: on a substrate of a diboride single crystal expressed by a chemical formula XB₂, in which X includes at least one of Ti, Zr, Nb and Hf, growing an (AlN)_(x)(GaN)_(1-x) layer (1<x≦1) from vapor phase within a temperature range of more than 400° C. and less than 1100° C. by an MOVPE method, and subsequently, growing a nitride semiconductor layer including at least one kind selected from among 13 group elements and As from vapor phase.
 22. The method of claim 21, wherein the thickness of the (AlN)_(x)(GaN)_(1-x) layer is within a range of 10 to 100 nm.
 23. A method for growing a nitride semiconductor, comprising: on the (0001) plane of a substrate of a diboride single crystal expressed by a chemical formula XB₂, in which X includes at least one of Ti, Zr, Nb and Hf, growing an AlN layer from vapor phase so that a deviation angle of a normal line of a surface of the substrate from a direction of the [0001] becomes 0.55 degrees or less, and subsequently, growing a nitride semiconductor layer including at least one kind selected from among 13 group elements and As from vapor phase.
 24. The method of claim 23, wherein the thickness of the AlN layer is within a range of 10 to 250 nm.
 25. A method for producing a semiconductor apparatus, comprising: eroding and removing a diboride single crystal substrate of a semiconductor apparatus obtained by the method for growing nitride semiconductor of claim 21 by etching.
 26. A method for producing a semiconductor apparatus, comprising: eroding and removing a diboride single crystal substrate of a semiconductor apparatus obtained by the method for growing nitride semiconductor of claim 22 by etching.
 27. A method for producing a semiconductor apparatus, comprising: eroding and removing a diboride single crystal substrate of a semiconductor apparatus obtained by the method for growing nitride semiconductor of claim 23 by etching.
 28. A method for producing a semiconductor apparatus, comprising: eroding and removing a diboride single crystal substrate of a semiconductor apparatus obtained by the method for growing nitride semiconductor of claim 24 by etching.
 29. A method for producing a semiconductor apparatus, comprising the steps of: carrying out crystal growth of a nitride semiconductor layer on one principal surface of a single crystal substrate of a hexagonal crystal symmetry having electrical conductivity; and eroding and removing the single crystal substrate by etching.
 30. The method of claim 29, wherein the single crystal substrate is a substrate of a diboride single crystal expressed by XB₂, in which X includes at least one of Zr and Ti.
 31. The method of claim 29, wherein in growing the nitride semiconductor layer from vapor phase, a nitride semiconductor layer grown firstly is an Al_(x)Ga_(1-x) layer (0<x≦1).
 32. The method of claim 29, wherein a mixed solution of at least nitric acid and hydrofluoric acid is used for the etching. 